The present invention is directed to the field of network management. The invention has particular applicability to the improvement of a desired performance metric in a wireless communication network. A typical wireless network includes a plurality of wireless clients such as laptop computers and cellular phones that exchange data packets with a network backbone via a radio card. These clients gain access to the network backbone by way of an access point (AP), such as a wireless base station, connected to the network by a hardwired connection or a fixed Ethernet connection.
The advantage of a wireless network is that clients are mobile units that can vary in position (such as with wireless laptop computers) or can even be in motion while in use (such as with cellular telephone service.) However, these wireless clients have only a finite range wherein the base station may be accessed. Beyond that range, a client cannot maintain a connection to the base station. Consequently, in order to cover a large area, a number of such access points are required so as to maintain service to the clients. At any given time, an AP serves a number of clients in a unit called a Basic Service Set (BSS) and the region served is called a Basic Service Area (BSA).
As mobile clients vary in position, each BSA has an ever changing topology of wireless connections. However, due to regulatory frequency allocations, there is not enough available spectrum to insure that each client can gain access to the network at the same time. A number of schemes are known to permit clients to share access through an AP within a BSA. However, it may happen that multiple BSA's can be on the same channel, producing interference from “packet collisions” that break the network, resulting in “packet drops”. Other reductions in quality-of-service (QoS) can occur such as packet delay and packet jitter, which can also lead to packet drops.
In order to reduce interference from packet collisions in a wireless network, access can be controlled using a distributed coordination function (DCF), which relies on “carrier sense, multiple access collision avoidance” (CSMA/CA). With DCF control, each AP and client radio listens to the air and waits a random amount of time for the air to clear. If the air is then clear, the radio transmits a packet. In this manner, network access is regulated through statistics. This technique is found to statistically provide a modest reduction in packet collisions, though not altogether preventing collisions. The principle drawback is that only one BSA can be on the air at a time, since frequency bandwidth is restricted by FCC regulations. Consequently, the network throughput of BSA's sharing the air is fixed and is divided by the number of radios on the common channel. As the number of AP's and clients sharing the air is increased, access is further diluted, resulting in increased packet collisions and a significant waste of bandwidth.
As shown in FIG. 1A a typical DCF control uses an AP with an omni-directional antenna 10, which transmits and receives in a 360 degree radiation pattern to access all the associated clients. As shown in FIG. 1B, the problems with DCF control are partially ameliorated by the use of an adaptive directional antenna 20 having a narrow radiation pattern 22 that can be varied so as to be selectively directed to each client in the BSA. The adaptive directional antenna 20 can be an array of antennas with rapid switching therebetween.
This adaptive directional antenna 20 can be used in a point coordination function control (PCF) to perform time division, multiple access (TDMA) rather than CSMA/CA. In fact, adaptive antennas are better used by PCF, which offers improvement in QoS. With PCF, every client 24 is assigned a time slot by the AP in a predetermined manner in which the AP queries the clients, and receives a packet or transmits a packet. PCF is superior DCF in that access can be regulated and prioritized to reduce packet drop outs from packet collisions and other QoS problems resulting from queuing issues, such as packet delays and packet jitter. PCF control works especially well for real-time communications such as voice traffic.
However, PCF control creates complications in a wireless network where multiple BSA's share the same channel. AP's work independently, with no communication between each other. Consequently, interference can still occur between radios in different BSA's, especially if they lie along the same line of sight. FIG. 1C shows adjoining BSA's A, B having respective AP's 20a, 20b, each connected to a network backbone 30. The AP 20a of BSA A can be in contact with a client 24a while in line of sight with a client 24b associated with the BSA B. Interference can result if both cells (AP's and clients) become active at the same time, especially if e.g. AP 20a transmits to client 24a while client 24b transmits to AP 20b. As the clients are mobile, the network topology is always changing.